A polymer solution can exhibit Lower Critical Solution Temperature (LCST) phenomenon, whereby a homogeneous polymer solution separates into a polymer rich liquid phase and a solvent-rich phase above a certain temperature. This temperature is a function of solvent type, polymer stream composition and pressure. Any of these variables can be manipulated to a induce liquid-liquid separation. This separation has very small heat duty associated with it, especially compared to the vaporization of an equivalent amount of solvent. In commercial solution polymerizations, there is a need to increase efficiency and reduce costs associated with solvent removal processes.
U.S. Pat. No. 6,881,800 relates to processes and plants for continuous solution polymerization. Such plant and processes include a pressure source; a polymerization reactor, downstream of said pressure source; a pressure let-down device, downstream of said polymerization reactor; and a separator, downstream of said pressure let-down device. The pressure source is disclosed as sufficient to provide pressure to the reaction mixture during polymerization to produce a single-phase liquid reaction mixture in the reactor, and a two-phase liquid-liquid reaction mixture in the separator, in the absence of an additional pressure source between the reactor and the separator. See also U.S. Pat. No. 7,163,989. This process discloses the use of a heater to heat the reactor outlet stream, prior to inducing liquid-liquid phase separation. Since the solution coming out of the reactor has more solvent per pound of polymer than that coming out of the separator, heating prior to the separator significantly increases the heat duty per pound of polymer.
International Publication No. WO 2008/076589 discloses a process for polymerizing olefins in a dense fluid, homogeneous polymerization system. The process comprises the following steps: (a) contacting, in one or more reactors, olefin monomers having three or more carbon atoms present at 30 weight percent, or more, with the following: 1) one or more catalyst compounds, 2) one or more activators, 3) from 0 to 50 mole percent comonomer, and 4) 0 to 40 weight percent diluent or solvent; (b) forming a reactor effluent comprising the polymer-monomer mixture; (c) optionally heating the polymer-monomer mixture of (b), after it exits the reactor, and before, or after, the pressure is reduced in step (e); (d) collecting the polymer-monomer mixture of (b) in a separation vessel; (e) reducing the pressure of the reactor effluent comprising the polymer-monomer mixture of (b) below the cloud point pressure to form a two-phase mixture comprising a polymer-rich phase and a monomer rich phase, either before, or after, collecting the polymer-monomer mixture in the separation vessel. The pressure in the reactor is from 7 to 100 MPa higher than the pressure in the separation vessel, and the temperature in the separation vessel is above the higher of the crystallization temperature of the polymer, or above 80° C., if the polymer has no crystallization temperature. The monomer-rich phase is separated from the polymer-rich phase, and recycled to one or more reactors. This patent discloses that a reactor pressure required to run this process with less than 40 weight percent solvent is high (up to 200 MPa), to ensure supercritical polymerization medium. This high pressure makes the reactor operation challenging, and requires the use of thick walled reactors which reduce capital and energy efficiency.
U.S. Pat. No. 6,255,410 discloses processes for producing polyolefins at pressures substantially below conventional high pressure conditions in two-phase systems. The process comprises the following steps: (a) continuously feeding olefinic monomer and catalyst system of metallocene and cocatalyst; (b) continuously polymerizing monomer feed to provide a monomer-polymer mixture; and (c) continuously settling a two phase mixture into a continuous molten polymer phase and a continuous monomer vapor, the latter of which may, optionally, be at least partly recycled to (a). In step (b), the mixture is at a pressure below the cloud point pressure to provide a polymer-rich phase and a monomer-rich phase at a temperature above the melting point of the polymer, and the polymerization takes place at a temperature and a pressure, where the catalyst system productivity exceeds that which is obtained at twice said pressure above the cloud point at that temperature. This patent discloses only minor amounts of solvent, as required for a catalyst carrier, and does not have the advantages of using lower temperatures and pressures associated with solution polymerization processes.
U.S. Pat. No. 4,444,922 discloses a method for processing a polymer solution by changing the thermodynamic state of the solution to produce a dilute phase and a concentrated phase by spinodal decomposition. This reference discloses a method for processing a conjugated diene polymer solution, at an elevated pressure and temperature, and comprising the step of rapidly reducing the pressure of said heated solution to a pressure sufficiently low to cause said solution to form, by spinodal decomposition, a first phase having a relatively low polymer concentration and a second phase having a relatively high polymer concentration. This patent discloses three methods to induce liquid-liquid phase separation, which all include adding heat to induce the liquid-liquid separation. Each methods is expensive and energy intensive, in part, since the entire reactor solution must be heated.
U.S. Pat. No. 4,433,121 discloses a polymerization that is carried out in a polymerization zone, at a temperature above the upper cloud point of said polymer solution, and under conditions which enable the polymer solution to be separated into two phases. The polymerization is also carried out under stiffing conditions, which maintain the two phases under said phase-separating conditions, in a dispersed and mixed state. The resulting polymer solution is sent to a separating zone, located independently of said polymerization zone, and is separated into two phases at a temperature above the upper cloud point. Thereafter, the polymer-rich liquid phase, as a lower layer, is recovered, while the polymer-lean liquid phase, as an upper layer, is recycled to the polymerization zone. There is an inherent challenge of operating the solution polymerization reactor in two liquid phase region. Since the polymer microstructure is determined by the components' concentrations in the reactor, the distribution of the components in both phases will have impact in the product composition and molecular weight. Subsequently, increased variability in the final product may result.
U.S. Pat. No. 5,599,885 discloses a polyolefin polymerization that is carried out in the presence of an aliphatic hydrocarbon diluent or an alicyclic hydrocarbon diluent having a boiling point below 100° C. In one embodiment, a polymer solution containing the resulting polyolefin is fed to a separation zone, kept at a temperature of not lower than the upper cloud point of the polymer solution, to separate the polymer solution into a lower phase portion, containing a high concentration of polyolefin, and an upper phase portion, containing a lower concentration of the polyolefin. See also EP0552945B1. The liquid-liquid separation achieved by raising temperature is not efficient, since there is a time limit on how fast the entire reactor content can be heated. Such a means of separation inevitably leads to at least partial liquid-liquid separation through a “nucleation and growth” mechanism rather than a spinodal decomposition.
U.S. Pat. No. 4,319,021 discloses a process for recovering a polymer from a solution of the polymer in a solvent, by a high temperature phase separation, in which a low molecular weight hydrocarbon is added to the solution, the solution and the added hydrocarbon are subjected to a temperature and pressure so that one liquid phase is formed. The pressure is reduced to form three phases, namely, a vapor phase rich in the hydrocarbon, a polymer-lean liquid phase, and a polymer-rich liquid phase. The polymer-rich liquid phase is separated from the vapor phase and the polymer-lean liquid phase, and the polymer is then recovered from the polymer-rich liquid phase. In this process, the light hydrocarbon solvent is added after the reactor to induce phase separation. This in turn requires a separation of the reactor solvent and un-reacted co-monomers and this added light hydrocarbon, for the process to run continuously. This separation can be difficult, particularly when the co-monomers have volatility that is close to that of the light hydrocarbon that is added to induce phase separation. Pressurizing the polymer solution, and heating it, before inducing phase separation, is very inefficient.
International Publication No. WO 2008/082511 discloses a process for fluid phase in-line blending of polymers. The process includes providing two or more reactor trains configured in parallel, a separator for product blending and product-feed separation. In at least one of the parallel reactor trains, olefin monomers having three or more carbon atoms, catalyst systems, optional comonomers, optional scavengers, and optional inert diluents or inert solvents, are reacted at a temperature above the solid-fluid phase transition temperature of the polymerization system, and a pressure no lower than 10 MPa below the cloud point pressure of the polymerization system and less than 1500 MPa, to form a reactor effluent, that includes a homogeneous fluid phase polymer-monomer mixture in each parallel reactor train. The reactor effluents from each parallel reactor are combined, and passed through the separator. See also U.S. Publication 2008/0234443.
International Publication No. WO 2008/109212 discloses a process to polymerize olefins, comprising contacting propylene, at a temperature of 65° C. to 150° C., and a pressure of 1.72 to 34.5 MPa, with: 1) a catalyst system comprising one or more activators and one or more nonmetallocene metal-centered, heteroaryl ligand catalyst compounds, 2) optionally one or more comonomers selected from ethylene and C4 to C12 olefins, 3) diluent or solvent, and 4) optionally scavenger. The olefin monomers and any comonomers are present in the polymerization system at 30 wt % or more, and the propylene is present in the feed at 80 wt % or more. The polymerization occurs at a temperature above the solid-fluid phase transition temperature of the polymerization system, and a pressure greater than 1 MPa below the cloud point pressure of the polymerization system, and the polymerization occurs at a temperature below the critical temperature of the polymerization system, or at a pressure below the critical pressure of the polymerization system.
International Publication No. WO 2008/079565 discloses a process to polymerize olefins, comprising contacting one or more olefin monomers having three or more carbon atoms, with a catalyst system comprising one or more activators and one or more nonmetallocene metal-centered, heteroaryl ligand catalyst compounds, 2) optionally one or more comonomers, 3) optionally diluent or solvent. The olefin monomers and any comonomers are present in the polymerization system at 40 wt % or more, and the monomer having three or more carbon atoms is present at 80 wt % or more, based upon the weight of all monomers and comonomers present in the feed. The polymerization occurs at a temperature above the solid-fluid phase transition temperature of the polymerization system, and a pressure no lower than 10 MPa below the cloud point pressure of the polymerization system, and less than 1500 MPa. If the solid-fluid phase transition temperature of the polymerization system cannot be determined, then the polymerization occurs at a temperature above the fluid phase transition temperature. See also U.S. Publication 2008/0153996.
International Publication No. WO 2008/109094 discloses a monomer recycle process for fluid phase in-line blending of polymers. The monomer recycle process includes providing a first group (G1) of one or more reactor trains, and a second group (G2) of one or more reactor trains, and one or more separators fluidly connected to G1 and G2. In each of G1 and G2 olefin monomers are polymerized to form homogenous fluid phase polymer-monomer mixtures, wherein each of the G1 and G2 has at least one common monomer. The reactor effluents from G1 are passed through the G1 separators to separate a monomer-rich phase from a polymer-enriched phase, and the polymer-enriched phase and the reactor effluents from G2 are passed into the G2 separator to separate another monomer-rich phase from a polymer-rich blend. The monomer-enriched phase is recycled.
Additional polymerization processes or polymer separation processes are disclosed in U.S. Pat. Nos. 3,781,253; 3,553,156; 3,726,843; 3,496,135; 4,857,633; 4,623,712; 4,319,021; 4,946,940; 5,264,536; 6,683,153; 7,629,397; 7,650,930; U.S. Publication Nos. 2009/0259005; 2008/0090974; 2008/0027173; 2008/0033127; 2007/0299161; 2007/0244279; European Patent Nos. EP0149342B1; EP0184935B1; Canadian Patent Applications CA 2372121A1; CA 1203348; and German Patent Application DE 19905029.
The polymerization processes described in the above references are typically energy intensive, requiring heat exchanges between the polymerization reactor and the separator, supercritical polymerization conditions, and/or additional polymer-solvent separation means. There is a need to develop new polymerization processes that use solvent separation means that require less energy to run, and have increased efficiency and reduced costs. There is also a need to eliminate ancillary and energy intensive devices, and thus, reducing capital and operating costs. These needs and others have been met by the following invention.